A seismic survey represents an attempt to map the subsurface of the earth by sending sound energy down into the ground and recording the "echoes" that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, and air guns in marine environments. During a seismic survey, the energy source is moved across the surface of the earth above a geologic structure of interest. Each time the source is detonated, it generates a seismic signal that travels downward through the earth, is reflected, and, upon its return, is recorded at a great many locations on the surface. Multiple explosion/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single straight line, whereas in a three-dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data "cube" or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area.
A seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces. General background information pertaining to 3-D data acquisition and processing may be found in Chapter 6, pages 384-427, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, the disclosure of which is incorporated herein by reference.
A modem seismic trace is a digital recording (analog recordings were used in the past) of the acoustic energy reflecting back from inhomogeneities in the subsurface, a partial reflection occurring each time there is a change in the acoustic impedance of the subsurface materials. The digital samples are usually acquired at 0.004 second (4 millisecond or "ms") intervals, although 2 millisecond and 1 millisecond sampling intervals are also common. Thus, each digital sample in a seismic trace is associated with a travel time, and in the case of reflected energy, a two-way travel time from the surface to the reflector and back to the surface again. Further, the surface location of every trace in a seismic survey is carefully recorded and is generally made a part of the trace itself (as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific subsurface locations, thereby providing a means for posting and contouring seismic data--and attributes extracted therefrom--on a map (i.e., "mapping").
The data in a 3-D survey are amenable to viewing in a number of different ways. First, horizontal "constant time slices" may be taken extracted from a stacked or unstacked seismic volume by collecting all digital samples that occur at the same travel time. This operation results in a 2-D plane of seismic data. By animating a series of 2-D planes it is possible for the interpreter to pan through the volume, giving the impression that successive layers are being stripped away so that the information that lies underneath may be observed. Similarly, a vertical plane of seismic data may be taken at an arbitrary azimuth through the volume by collecting and displaying the seismic traces that lie along a particular line. This operation, in effect, extracts an individual 2-D seismic line from within the 3-D data volume.
Seismic data that have been properly acquired and processed can provide a wealth of information to the explorationist, one of the individuals within an oil company whose job it is to locate potential drilling sites. For example, a seismic profile gives the explorationist a broad view of the subsurface structure of the rock layers and often reveals important features associated with the entrapment and storage of hydrocarbons such as faults, folds, anticlines, unconformities, and sub-surface salt domes and reefs, among many others. During the computer processing of seismic data, estimates of subsurface velocity are routinely generated and near surface inhomogeneities are detected and displayed. In some cases, seismic data can be used to directly estimate rock porosity, water saturation, and hydrocarbon content. Less obviously, seismic waveform attributes such as phase, peak amplitude, peak-to-trough ratio, and a host of others, can often be empirically correlated with known hydrocarbon occurrences and that correlation applied to seismic data collected over new exploration targets. In brief, seismic data provides some of the best subsurface structural and stratigraphic information that is available, short of drilling a well.
That being said, unprocessed seismic data is only of limited use to an explorationist. Seismic data as it is acquired in the field is seldom used directly, but instead it is first returned to a processing center where various mathematical algorithms are applied to the digital seismic data to enhance its signal content and generally make it more amenable to interpretation. A key step in a typical seismic processing sequence is seismic migration, or inversion as it is also characterized.
As is well known to those skilled in the art, the dip and location of a reflector on an unmigrated seismic section is rarely representative of the true dip and subsurface location of the structural or stratigraphic feature that gave rise to that reflector. Except in the case where the subsurface consists of homogenous nearly-horizontal layers, the recorded seismic expression of a structural or stratigraphic event must be migrated before it can be reliably used to locate subsurface features of interest. In areas of steep dip, a reflection that is apparently located directly below a particular shot point may actually be found several hundreds of feet laterally away from that shot point. Additionally, in complex structural areas where faulting, severe asymmetrical folding and sharp synclines are present, diffractions and multiple reflections may interfere with reflections from the primary reflectors to the point where the resulting seismic section bears little or no resemblance to the actual subsurface structure.
Broadly speaking, migration improves a seismic section or volume by "focusing" the seismic data contained therein, a process that is conceptually similar to that of "focusing" the image produced by a slide projector in order to obtain the sharpest screen image. Migration improves the seismic image by correcting the lateral mispositioning of dipping seismic reflectors; collapsing diffractions caused by point scattering centers and subsurface fault terminations; resolving crossing reflectors (conflicting dips); and improving the vertical and lateral resolution of the seismic data, among many others. A general description of the many ways that migration improves seismic data may be found in, for example, Chapters 4 and 5, and Appendix C, pages 240-383, and 507-518, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, the disclosure of which is incorporated herein by reference. As a general rule, seismic data that have been properly migrated reveal an enhanced or a truer picture of the subsurface than unmigrated seismic data, the ultimate goal of seismic migration being to produce a seismic section or volume that accurately represents the configuration of the geology of the subsurface.
There are two broad variants of seismic migration: migration after stack (post-stack) and migration before stack (prestack). Post-stack migration is applied, as the name suggests, to seismic traces after they have been stacked--a stacked seismic trace being one that is formed by combining together two or more traces to form a single composite trace. Prestack migration, on the other hand, is applied to seismic traces before they have been stacked. Other things being equal prestack migration is always preferred--both theoretically and in practice--because it has the potential to produce a more accurate picture of the subsurface stratigraphy and structure. However, the computational effort involved in computing a prestack migration is many times that required to do a post-stack migration. For 2-D seismic lines, this additional computational effort is generally manageable and, except for the longest lines, prestack migration is often applied to seismic data that has been taken in areas where the subsurface structure is complicated. Most 3-D data sets, on the other hand, contain far too many traces to be migrated via conventional prestack algorithms.
A further division of seismic migration algorithms may be made based on whether the migration takes place in "depth" or "time." Broadly speaking, the operational difference between the two approaches is that a trace in the seismic volume that results from a depth migration contains digital samples that are separated by units of depth, rather than time. In more particular, whereas a seismic trace in a conventional seismic survey has samples that are spaced some distance .DELTA.t apart in time (where, for example, .DELTA.t might be 4 ms), successive samples in a depth migrated volume are spaced some depth, say .DELTA.z, units apart. Prestack and post-stack migrations may be performed either in time or in depth. Finally, it is well known to those skilled in the art that seismic data that have been migrated in depth may be easily transformed into a conventional "time" section and vise versa.
It is necessary to define a velocity model before a seismic migration can be performed. A velocity model is just a specification of the subsurface velocity structure as a function of depth for subsurface points located in the vicinity of the survey. The velocity model might be simple (involving only a few layers) or complex, but it has always been the conventional wisdom that the best velocity model is the one that most accurately represents the actual configuration of the rock units in the subsurface. In fact, the modern trend has been to move toward increasingly accurate velocity models in an effort to improve the final migrated product. However, it is also well known that useful--even if not completely accurate--results may be obtained when the velocity model is just a gross approximation to the actual subsurface velocity field. In fact, even migrations that use a "layer cake" or horizontally layered velocity model can yield useful results in many circumstances.
The most popular methods of migration in use today are all based ultimately on a solution to the wave equation. As is well known to those skilled in the art, the wave equation is a second order differential equation that describes seismic wave propagation in the subsurface. To the extent that this equation accurately represents seismic wave propagation in the subsurface--and to the extent that the true subsurface velocity and other rock parameters are correctly specified--this equation can be used migrate seismic data with considerable accuracy, a solution to the wave equation producing the theoretically correct way to migrate seismic data. Of course, for most media the wave equation cannot be solved exactly, thus some degree of approximation must be introduced into the solution. As a general rule, the better/more precise the approximation is to the true solution, the better the resulting migration. As might be expected though, the more accurate the approximation the greater the amount of computer time required to perform the migration. The most accurate migration methods are those that take into account--and recover--amplitude variations due to spreading, angle dependent reflections, attenuation, etc. Of course, implementing a migration algorithm that accounts for these sorts of transmission effects requires additional computer time beyond that normally required to migrate a seismic section. Thus, the most desirable migrations are those that use the highest fidelity wave transmission model, the disadvantage of this sort of models being that the computer time necessary to calculate a migration is lengthened accordingly.
The most common approaches to wave equation (based) migration are finite-difference methods, frequency domain (f-k) approaches, and Kirchhoff integral migration. Each of these methods has its advantages and disadvantages, one discussion of the various tradeoffs being found in Migration--The Inverse Method, by J. D. Johnson and W. S. French, which is published as chapter 5 (pages 115-157) of CONCEPTS AND TECHNIQUES IN OIL AND GAS EXPLORATION, Jain and de Figueiredo editors, Society of Exploration Geophysicists, 1982, the disclosure of which is incorporated by reference.
Seismic migration is a computationally intensive process, even for simple velocity models. In the case of 2-D data, modern computer speeds (coupled in some cases with the use of parallel and massively parallel processors) have made post-stack migration a routine processing step for most seismic lines, and prestack migration, although correspondingly more costly computation-wise, is also done rather routinely. However, the computational costs associated with a full prestack migration of 3-D seismic data are so enormous that it is seldom performed, despite the fact that many surveys would profit from its use. By way of example, a 3-D survey might consist of one million or more CMPs (e.g., a grid of 1024 CMPs by 1024 CMPs), each of which might be 40 to 60 fold. Thus, as many as 40 to 60 million unstacked seismic traces might be collected in a typical 3-D survey. Although prestack depth migration at "full fidelity" is the preferred approach, the computations involved in such a 3-D prestack migration might require six months or even more to complete at current computer speeds, thereby making it impractical for all but the most important seismic surveys. As a consequence, even a large seismic processing center might be unable to complete more than two or three 3-D prestack migrations a year, even if the computers are utilized at full capacity. This is in spite of the fact that 3-D data can potentially profit the most from prestack migration.
Heretofore, as is well known in the seismic processing and seismic interpretation arts, there has been a need for a method of prestack migration that is highly efficient. Additionally, the method should provide for a highly accurate migration at minimal additional computational cost. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.